In diploid cells of Saccharomyces cerevisiae, genetic recombination and recombinational repair are controlled in part by the mating-type locus (MAT). Diploids heterozygous (a/alpha) at MAT exhibit greater mitotic recombination and resistance to MMS and X-rays than homozygous MAT (a/a or alpha/alpha) diploids. The a/alpha-dependent DNA repair pathway is active largely in the S and/or G2 phases of the cell cycle, whereas MAT-independent repair can occur throughout the cell cycle. Part of the DNA repair capacity of a/alpha diploids is inducible, possibly by DNA damage. One or more nucleases whose synthesis/activity is influenced by MAT will be identified by comparisons of enzymatic activities in fractions from extracts of a/alpha and alpha/alpha diploids. The biological roles of such enzymes and of the a/alpha-dependent repair system in general will be assessed by studying various radiation-sensitive or mating-type mutants and a mutant alpha/alpha strain that has a/alpha repair, which we have isolated and characterized. We are also continuing our investigation of the genetic, physiological, and biochemical properties of a group of pleiotropic mutants with altered MAT regulation in order to gain more understanding of this complex system and locus. One part of MAT control in haploids includes the production of and response to mating hormones that are involved in the conjugation process. Physiological alterations which occur in hormone-treated cells may be useful assays in attempts to determine if the primary target for the hormone is extracellular or intracellular.